Troughed strip monitoring type speed control for continuous etcher

ABSTRACT

Automatic speed control apparatus is disclosed which monitors the degree of material removal from a control element strip fed thereto while the strip is being etched away and which, in response to the degree of material removal sensed thereby, and controls the conveying speed of articles being conveyed while being etched in a conveyorized type continuous etcher. The fed strip is thin stock preferably troughed for longitudinal stiffness against buckling and, when push-fed through a spray etching chamber, dissolves away to a point of discontinuity which is reached at or about the location of a monitoring or sensing point in the chamber where a sensing mechanism senses that the body of the strip has or has not undergone the last degree of material dissolution.

United States Patent f Benton et al.

[54] TROUGHED STRIP MONITORING TYPE SPEED CONTROL FOR CONTINUOUS ETCHER [72] Inventors: Robert C. Benton, State College; Blair W.

[21] Appl. No.: 2,574

[52] U.S. Cl ..156/345, 134/57 [51] Int. Cl ..H05k 3/00, B23p 15/00 [58] Field of Search 156/345; 134/57 [56] References Cited UNITED STATES PATENTS 3,388,023 6/1968 Benton etal ..156/345 [15] 3,657,09 i451 Apr. t8, 1972 3,475,242 lO/ 1969 Radimer 156/345 Primary Examiner-J. Steinberg Attorney-Paul & Paul [57] ABSTRACT Automatic speed control apparatus is disclosed which monitors the degree of material removal from a control element strip fed thereto while the strip is being etched away and which, in response to the degree of material removal sensed thereby, and controls the conveying speed of articles being conveyed while being etched in a conveyorized type continuous etcher. The fed strip is thin stock preferably troughed for longitudinal stiffness against buckling and, when push-fed through a spray etching chamber, dissolves away to a point of discontinuity which is reached at or about the location of a monitoring or sensing point in the chamber where a sensing mechanism senses that the body of the strip has or has not undergone the last degree of material dissolution.

8 Claims, 8 Drawing Figures 3,657, 0&9

EATENTEHAPR 18 ma SHEET 2 UF 3 ollz/'enamf Robert C. enon Blair Z/ /ef'fzer llll IIL IlllllllllllllinIlllllql LII IIII Weffnr MTENTEDAPR l 8 |972 l SHEET 3 of 3 Blair m BACKGROUND OF THE INVENTION l. Field of the Invention This invention relates to the continuous etching of articles being advanced through an etching machine by means of a steadily running drive motor for article conveying means with which the machine is equipped, and to an automatically operating conveyor control. The automatic control is a speed control, which control senses the degree of material removal from a control element strip that is likewise being steadily advanced while being etched, and which control automatically controls the conveying means in response to the degree of material removal sensed, by regulating the speed of the drive motor. The control effected is known as the analog type.

2. Description of the Prior Art Metallic or metalized control elements have been used in several forms as the means of introducing etchable material into an analog type speed control. The material of the control element is etched to destruction and monitored as a measure of the degree of actual material removal from articles being simultaneously etched in an etcher. ln one successful operation the material has been fed into the control in the form of a continuous length of straight metal wire, and it has in other instances taken the form of an interrupted or a continuous length of structurally weak foil or film laminated to or coated upon a substrate which provides the necessary structural strength as the carrier.

The solid wire has the advantage of self sufficient strength to be push-fed through a guide, whereas the inherent thinness of the foil or film strip gives it the advantage of fast action for breakthrough and easy and accurate guiding for monitoring purposes. Our invention provides a control element wholly consisting of a guided, permanently troughed thin metal strip which is structurally self sustaining so as not to buckle in the guide while being consumed and, in the process, is completely consumed, so that there can be no wastage or scrappage substrate because a carrier for the strip is neither necessary nor desirable.

SUMMARY OF THE INVENTION This invention provides automatic speed control apparatus used in combination with a conveyorized type continuous etcher. The control element is an unbacked control strip which is permanently troughed for stiffness by a pair of rolls, one of which has a protruding peripheral portion and f'its in a concave portion of the other roll to trap, feed, and permanently defonn the control element therebetween. The element is fed through a guide having shields at the edgesvsuch that the center portion of the element is exposed to etchant while the marginal edges are not so exposed. The control element thus exposed to etchant thins down to a point of discontinuity at an end of the guide channel generally in registry with a monitoring mechanism, the essentially intact marginal remainders of the guided element extending past the point of discontinuity. The monitoring mechanism senses the point of discontinuity and thereby monitors the etched-through condition of the passing control element.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is an isometric view from inside a continuous etching machine embodying the present control invention.

FIG. 2 is a cross sectional view in rear elevation of the idler and capstan rolls of FIG. 1, and taken along the section line lI-II of the latter.

FIG. 3 is a schematic diagram of the speed control circuit of the etching machine.

FIG. 4 is a cross sectional view in right side elevation of a modified form of the invention.

FIG. 5 is a top plan view ofthe rolls of FIG. 4.

FIGS. 6 and 7 are respective cross sectional rear elevation and bottom plan views of the strip guide of FIG. 4, FIG. 6 being taken along the section line VI-Vl of FIG. 4.

FIG. 8 is an isometric view from the rear, showing the strip guide with a deflector added.

DESCRIPTION OF THE PREFERRED EMBODIMENT More particularly in FIG. 1 of the drawings, a continuous spray etching machine l0 is shown having a fluid tight spray chamber 12 provided with the usual six walls, of which the front wall 14 and left side wall 16 are illustrated. The chamber contains vertically directed spray nozzles including rows of upper nozzles 18 which direct pressure induced sprays of liquid etchant downwardly.

Continuously running conveying means is provided for carrying articles to be etched longitudinally through the etcher and, in the various types of etchers contemplated for using our invention, the articles being etched can be in a series entirely separate from one another or together in a common web from which they are ultimately separated into individual articles after etching. The articles for the broad purposes of this disclosure are either carried in a vertical disposition when conveyed longitudinally through the etcher as aforesaid, or carried horizontally in which case the etcher used is one of the types called a horizontal etcher. Examples of the articles are printed etched circuits, perforated sheets, gridwork, and flat springs, all of which are coated in selected areas with an etch resist.

The etcher as actually illustrated is of the horizontal type having a horizontally disposed roller conveyor 20. In operation of the conveyor, the articles being introduced into the etcher move thereon from a loading station, indicated by an arrow 22, thence in the longitudinal direction of the arrow through a suitably covered opening, not shown, in the chamber front wall 14, thence through the chamber 12 and out the chamber rear wall, not shown, so as to continue on the conveyor through the rest of the etcher for further treatment including rinsing.

All rollers in the conveyor 20 turn at the same speed and in the same direction, being fixed in groups on a series of parallel, power driven roller shafts 24 which are joumalled crosswise in the etcher in a common plane. A hollow, longitudinally extending conveyor side rail 26 is xed adjacent the chamber left side wall 16 and houses in its box-shaped interior a walking beam 28 which constantly swings in an endless closed path in a vertical plane inside the side rail. Each shaft 24 has, at least at the end thereof adjacent the walking beam 28, a journal portion supported at that end in a bearing hole 30 in the side rail and an integral crank portion, not shown, supporting at that end the walking beam so that the latter can swing.

A conveyor drive motor 32, coupled thereto by a reduction drive including gearing 34, directly drives some selected one of the roller shafts such as the shaft 24a having the identifying subscript, which in turn drives the walking beam indicated at 28 on one side of the machine and also drives a companion walking beam which it will be understood is provided on the opposite side ofthe machine.

The conveyor 20 has an automatic speed control which provides automatic control thereover by regulating the speed of the conveyor motor 32, and novelty is felt to reside in the coacting components of such control which, as disclosed herein, include a supply 36 of control element strip 38 in a particular form, a push-feed 40 for the strip, a stationary guide 42, a fiber optic monitoring device 44 for the strip, and a control circuit having, among others, photocell components 46 connected in a line 48 of the control circuit.

Specifically, a generally upstanding post 50 which supports the strip supply 36 is located at the inlet station of the etcher in an out of the way point atop the side rail 26. The strip 38 unwinds from a roll in the supply 36, and consists of a narrow width of thin, unbacked metal foil preferably having the same etching characteristics as the material being etched from the articles themselves. As an example, the strip is desirably a copper alloy or copper metal when the articles are copper or copper clad. The strip material contains metal usually, but not necessarily.

The push-feed 40 receives the strip 38 in flat form and troughs it into a permanently deformed, transversely arcuate cross sectional shape of constant radius as defined between forming rolls comprising an upper idler 52 and a lower capstan S4. The capstan 54 has a central protrusion at the periphery, and the idler 52 is annularly complementarily grooved at the periphery so as to interfit with the protrusion.

One of the roller shafts 24h identified by its distinguishing subscript is connected by a gear train 56, as indicated on FIG. 1, to drive the capstan 54 in the direction of the conveyor rol- Iers, at a proportional speed thereto. Hence, the troughed strip 38 moves at a fixed speed ratio to movement of the articles through the chamber 12.

The stationary guide 42 has an inlet operatively in registry with the gap between the rolls of the push-feed 40 and conducts the strip through a covered opening in the front wall 14 so that the strip enters into and is etched to destruction inside the spray chamber. The guide has narrow, inwardly facing upstanding shields 58 at opposite sides of a channel base 60 which define inwardly facing slots 62 engageable with and guiding the control element strip 38 at the strips opposite edges.

Etchant applied by the nozzles 18 dissolves the strip 38 at the upper exposed side of its body between the opposite edges thereof so that the central body section thins down to a point of discontinuity at 38a. The strip at that point assumes a bifurcate shape of which the opposite generally intact side edges 38b form the respective parallel bifurcations.

Troughing is optional with control element strips of thicker cross sections which have self sufficient columnar strength not to buckle when pushed through the stationary guide. In the case of thin strip, however, the combined feed-and-forming rolls hereof are essential because transverse troughing is necessary to insure the proper longitudinal stiffness for feedmg.

The cover for the opening in the chamber front wall 14 prevents spray from splashing to the outside of the chamber and includes a stationary channel 63 and a telescopically interfitting movable channel 64. The channel 64 has positions dictated by a guide block 66 which increase or decrease the effective length of the trough strip portion exposed to etchant spray. The adjusted position is selected externally by means of a lead screw 68 threaded through the block 66, and the adjustment compensates for such variables encountered in the field as strip thickness compared to the material thickness of the articles being etched, strip speed compared to article speed, the comparative etch rates between the strip and articles, and the comparative overall lengths of available travel in the chamber 12 as between the strip and the articles.

Articles are usually treated in the machine for a particular percentage of overetch, in which case the effective length of exposed strip 38 is adjusted so that the point of discontinuity 38a will be occurring in the right place, at the same time at which the articles leaving the chamber have remained therein long enough to acquire the particular percentage of overetch desired.

The channel base or bottom 60 is preferably but not necessarily curved precisely complementarily to the arc of curvature of the unexposed concave side of the troughed strip 38. Blow off air is introduced by a pipe 70 at a point between the closely fitting channel base and the strip 38 to prevent etchant from leaking in and attacking the strip at the unexposed back side.

Ideally, the discontinuity 38a stabilizes at a point falling between a pair of longitudinally aligned first and second monitoring openings 72 and 74 formed transversely in the channel base 60. The fiber optic device 44 comprises two bundles of light conducting fibers, one of which being on opposite sides of the strip at the location of first monitoring opening 72 and the other of which being on the opposite sides of the strip at the location of second monitoring opening 74. A common lamp 76 and a lens 78 register at one end of the bundles and the individual photocell components 46 register at the other end of the respective bundles. Thus the point of strip discontinuity, known as the breakthrough or breakout point, is optically monitored with photosensing means externally of the spray chamber so that the article speed can be accurately controlled by the automatic control circuit for the conveyor.

ln the control schematic of FIG. 3, power from a set of input leads is transformed to low voltage alternating current and applied to the lamp 76, the power is also applied by an SCR (silicon controlled rectifier) power supply circuit 82 to the conveyor drive motor 32, and the power is applied by a differential amplier circuit 84 to an SCR-operated reversible servomotor 86.

By a mechanical connection suitably provided, the drive motor 32 carries an electric tachometer 88 on the shaft therewith, the output of the tachometer 88 being adjustable and applied by a feedback loop circuit 90 to control the SCR power supply circuit 82 for the drive motor 32. The rotational position of the servomotor 86 is transmitted by a connection 92 to mechanically position the slider 94 of a potentiometer which adjusts output just referred to from the electric tachometer 88.

In brief, light from the lamp 76 falling upon the photocell components 46 controls the latter, and the photocells in the line 48 in turn control the differential amplifier 84 which sets the position of the servomotor 86. The mechanical connections to the tachometer and to its output potentiometer slider from the respective motor 32 and servomotor 86 enable the tachometer, by its automatically varied output, to regulate the conveyor by sensing the speed of the drive motor 32 and changing it to the proper value for the etching conditions.

The above sequence of operations is entirely automatic once the circuit is properly adjusted for the control factors.

The adjustment is readily made by allowing full light from the lamp 76 to fall upon the more remote one of the photocells 46, which is connected to monitor opening 74, and by blocking light from the other photocell connected to monitoring opening 72. This corresponds to the condition where the point of discontinuity lies between the two monitors. The slider 96 of a potentiometer connected to line 48 is adjusted so that equal biasing currents are introduced, through lines 196 and 98 respectively, into the two branches of the differential amplifier circuit 84. Upon balance of the two branches, a no output, null condition results, and the servomotor holds its position. It is apparent, however, that if the point of discontinuity changes such that either the light level decreases on the more remote, illuminated photocell, or the covered and darkened photocell receives light, the null condition will cease, causing the servomotor 86 to change its position one way or the other.

The conveying speed will accordingly increase or decrease with changes in light, such changes being directly controlled by the point at which discontinuity is being reached in the steadily advancing and dissolving control element strip.

Or, more to the point, the additional blocking of light by the control element strip is indicative of a slowed down etching rate and the need for systematically slowing down the conveying means. Hence, the conveyor continues to slow both the article movement and the strip movement until the effective length of the strip shortens and the breakthrough point shifts to the desired null-creating or midway position relative to the monitoring opening means 72 and 74, FIG. l. Conversely, increased uncovering of the photocell light source by the strip indicative of premature breakthrough of the control element strip causes increasing speed of the conveyor until the breakthrough point can be delayed until it has advanced in the direction of strip travel to about midway between the monitoring opening means 72 and 74.

Various information on specific details of the connections of the circuit and on the operation of individual components thereof can be found in the same assignees U. S. Pat. No. 3,388,023, the disclosure of which is incorporated in entirety herein by reference.

In the preceding embodiment, the etcher control is primarily intended to be factory installed on original equipment, and it is desired in the horizontal etcher disclosed that the exposed face be the top surface of the control element strip. But it is not always desirable to expose the top surface, however, and in the instance of a vertical etcher the longitudinally moving control element strip therefor would be disposed more or less in a vertical plane for receiving laterally directed spray against one side face only.

As a further example, the control element .strip receives spray on an exposed bottom surface in the embodiment illus trated in FIGS. 4 to 8, inclusive. In that embodiment the control is more universal in its adaptability, because the etch cell is self-contained to afford other applications including retrofit to etchers already in the field.

More particularly in FIGS. 4 to 8, the test cell or etch cell 100 is adapted to be secured to the outside of an etcher while the latter is either being finished at the factory or converted in the field. There are several things, not shown, which the cell 100 requires generally in common to the etcher, including the same etchant under pressure, a supply of control element strip made of material having similar etching characteristics as articles in the etcher, and one or more drives from the etcher maintaining the same or at least a proportional nozzle oscillation rate, if any, and maintaining the strip feed rate in the cell proportional to conveying rate in the etcher.

The cell 100 is a fluid tight cubicle having six walls of which a rear wall 102, a front wall 104, a top wall 106, and a right side wall 108 are shown. The rear wall is provided with a sealed light transmitting window 110 centrally thereof.

A horizontally disposed, centrally open mask 112 intervenes between an upspraying set of vertically disposed nozzles 118 and the upper portion of the cell 100 so as to keep the spray directed generally in a vertical plane containing the control element strip 138. The other components which supply and handle the strip do so in a manner fairly self-evident at this point, and are only briefly described below.

A roll of the flat metal strip 138 forms the supply 136 and is supported to feed by means of a fixed diagonal arm 150. A peripherally grooved lower capstan roll 152 and an upper idler roll 154 having a protuberant periphery permanently deform to a radius the unwinding strip 138 so as to impart an upwardly open trough shape thereto. A conveyor drive motor 132 in the etcher is coupled to a gear box 134 which directly drives the etcher conveying means, not shown, and which directly or indirectly through the conveying means drives the capstan roll 152.

Mounted within the cell, a stationary guide 142, which extends longitudinally and is generally the shape of a rail in its vertical attitude, has a pair of lengthwise extending bottom shields 158 at opposite sides of a centrally crowned channel base 160, all complementarily receiving the upwardly open trough shaped strip. Beside the channel base at the lower lateral side of the guide 142, a threaded block 166 receives a lead screw 168 which is fixed against axial movement and which provides for external fore and aft adjustment of the block. The block 166 carries a moveable flexible shield or baffle element strip 164 that is trained for movement in a part curved and part straight baffle guide 162 which confronts the channel base 160. The baffle strip 164 presents a length thereof co-extensive with the admitted portion of the strip 138 in the inlet of the guide 142, so that the effective shielding length of the baffle determines the effective length of the strip exposed to etchant from the spray nozzles 118. The inlet registers with an outside slit in the front wall 104.

An optical monitoring device includes a pair of resistor type photocells 146 and 148 which register in that order with a respective slitted monitoring opening and round monitoring opening in the channel base. The location of the openings is in a portion of the channel base 160 which extends at one end slightly past the foreshortened shields 158 at that end.

If desired, the extreme end ofthe guide 142 can be provided with a deflector plate having depending lugs 161 causing the bifurcations l38b of the strip to be broken o or to be bent and proceed downwardly as shown in FIG. 8. Such bifurcations are completely dissolved irrespective of whether they break off and fall directly into the etchant pool or not and, in the process, the downward diversion thereof prevents scratching the rear wall window 110.

In operation, a lamp 176 is a back mounted lamp box 178 in registry with the rear window directs light past the strip discontinuity 13811 (FIG. 7) onto the photocells 146 and 148. Preferably, the discontinuity occurs such that at the null point the strip bisects the slit thereunder, leaving it half blocked and half unblocked so that the photocell 146 receives what can be termed half light. The photocell 148 receives full light so that it continuously monitors the lamp and, by means of a circuit controlling the lamp input voltage, automatically maintains the light level constant irrespective of line voltage fluctuations, or bulb filament aging within the lamp 176, or etchant spray interference.

The servocontrol circuit which is provided is the one disclosed in the same assignees U. S. Pat. No. 3,388,023, the dis' closure of which is incorporated in entirety herein by reference. Briefly, increased blocking of the light by the strip as sensed by the photocell 146 causes the control circuit to slow the articles by progressively decreasing speed of the drive motor 132. Increased uncovering of the photocell light source by the strip indicative of premature breakthrough of the control element strip causes the conveying means to continue to speed up the article movement and the strip movement until the breakthrough point can be delayed until it has advanced in the direction of strip travel to about the half-light-producing position causing null over the slot opening. Ideally, the servocontrol circuit, not shown, stabilizes itself in the null condition and stabilizes the conveying means so that it continues at a constant running speed. Under such condition, no adjustment in conveying speed will occur as long as there is no change in the etch rate.

From the foregoing, and irrespective of whether the strip is disposed so as to be etched from the top or bottom surface or from a side facing surface, it is apparent that the strip body portion is reduced to breakthrough from one face only, and that the bifurcations continue alone at opposite sides from and past the monitoring openings. The continued integrity of the individual bifurcations after the body portion weakens due to thinning is necessary because the bifurcations pilot and support the fragile body and remain essentially intact during the terminal period to provide the structural strength. Although air blow off is a preferred expedient to prevent etchant from puddling and blocking the light source 176 or etching the unexposed side of the strip, as by blowing in air from an air pipe as shown in FIG. 4, protection against the etchant can be equally well afforded in other ways such as by applying a back pre-coating of oil or other etch resist to the strip.

What is claimed is:

1. Control apparatus operative to destructively test and monitor an unbacked moving strip of spray etchable control element material so as to derive signals useful for control of a spray etch chamber, said control strip having the general form of a shallow concavo-convex trough which is etched to breakthrough by spraying it on one side, comprising:

a. a strip channel for receiving and carrying said control strip, said channel arranged to block off a first one of the sides of said concavo-convex strip and to expose the op` posite side of said strip;

b. monitoring means for monitoring the point of breakthrough of said control element and for deriving said signals, positioned substantially normal to said strip channel and generally in registry with said point of breakthrough;

c. spray means in confronting relation to the exposed side of said strip channel for impinging etchant spray onto said control strip;

d. guide and shield means on said strip channel to slideably receive opposite edges of said troughed strip, having a pair of receiving slots disposed generally diagonally and inwardly facing, for supporting, guiding and shielding said strip at said opposite edges, said slots extending from an inlet of said guide and shield means to a point on the other side of said monitoring means from said inlet;

e. push-feed means to advance said strip from said inlet toward and past said monitoring means, said push feed means constituting the sole means for moving said strip; and,

f. said guide and shield means slideably supporting said strip which is intact prior to said point of breakthrough, said pair of slots shielding and slideably carrying essentially intact marginal remainders of said opposite edges of said strip which extend past said point of breakthrough, said push-feed means and said guide and shield means cooperating to continuously feed said unbacked strip past said point of breakthrough.

2. The control apparatus described in claim 1, wherein said monitoring means includes a pair of photocells.

3` The apparatus as described in claim l, said channel characterized by having the bottom thereof blocking oi the concave back side of said trough and presenting substantially complementary shape to the latter as viewed in cross section.

4. The apparatus as described in claim l, characterized by said channel having air jet means in the base thereof to blow etchant off the confronting rst side of said control element.

5. The apparatus as described in claim 4, in combination with said trough and characterized by said trough having a protective coating on the concave back side preventing etchant attack.

6. The apparatus as described in claim l, said push-feed means characterized by idler and capstan rolls establishing cooperation to permanently deform the continuous control element into a trough from a continuous flat strip of thin metal so as to double as fonning rolls while simultaneously pushing the trough as aforesaid.

7. The apparatus as described in claim 1, comprising: a shield adjacent the guide inlet and adjustable so that when the shield covers a portion of the length of the guide inlet it simultaneously shields a co-extensive portion of the control element.

8. The apparatus as described in claim 7, wherein said adjustable shield is characterized by a flexible flat element intervening across the face of the exposed second side of the control element. 

1. Control apparatus operative to destructively test and monitor an unbacked moving strip of spray etchable control element material so as to derive signals useful for control of a spray etch chamber, said control strip having the general form of a shallow concavo-convex trough which is etched to breakthrough by spraying it on one side, comprising: a. a strip channel for receiving and carrying said control strip, said channel arranged to block off a first one of the sides of said concavo-convex strip and to expose the opposite side of said strip; b. monitoring means for monitoring the point of breakthrough of said control element and for deriving said signals, positioned substantially normal to said strip channel and generally in registry with said point of breakthrough; c. spray means in confronting relation to the exposed side of said strip channel for impinging etchant spray onto said control strip; d. guide and shield means on said strip channel to slideably receive opposite edges of said troughed strip, having a pair of receiving slots disposed generally diagonally and inwardly facing, for supporting, guiding and shielding said strip at said opposite edges, said slots extending from an inlet of said guide and shield means to a point on the other side of said monitoring means from said inlet; e. push-feed means to advance said strip from said inlet toward and past said monitoring means, said push feed means constituting the sole means for moving said strip; and, f. said guide and shield means slideably supporting said strip which is intact prior to said point of breakthrough, said pair of slots shielding and slideably carrying essentially intact marginal remainders of said opposite edges of said strip which extend past said point of breakthrough, said push-feed means and said guide and shield means cooperating to continuously feed said unbacked strip past said point of breakthrough.
 2. The control apparatus described in claim 1, wherein said monitoring means includes a pair of photocells.
 3. The apparatus as described in claim 1, said channel characterized by having the bottom thereof blocking off the concave back side of said trough and presenting substantially complementary shape to the latter as viewed in cross section.
 4. The apparatus as described in claim 1, characterized by said channel having air jet means in the base thereof to blow etchant off the confronting first side of said control element.
 5. The apparatus as described in claim 4, in combination with said trough and characterized by said trough having a protective coating on the concave back side preventing etchant attack.
 6. The apparatus as described in claim 1, said push-feed means characterized by idler and capstan rolls establishing cooperation to permanently deform the continuous contRol element into a trough from a continuous flat strip of thin metal so as to double as forming rolls while simultaneously pushing the trough as aforesaid.
 7. The apparatus as described in claim 1, comprising: a shield adjacent the guide inlet and adjustable so that when the shield covers a portion of the length of the guide inlet it simultaneously shields a co-extensive portion of the control element.
 8. The apparatus as described in claim 7, wherein said adjustable shield is characterized by a flexible flat element intervening across the face of the exposed second side of the control element. 